Add functionality to save and restore recently used Emojis

Update emoji and add new categories based on AOSP's XML file

Fixes #634

by making it only selectable in detail view

This also adds comments to the blog post detail screen that were
previously missing.

Comments and reblogs need to depend on the post they refer to.
Since message dependencies are limited to one group,
the post and also the comments need to be wrapped
when commented on or reblogged to another blog.

For this reason, in addition to comments, two new wrapping message types
are introduced. They retain all data of the original messages and allow
for reconstruction and signature verification.

This commit breaks backwards compatibility with old blog posts.
It removes the content type, title and parent ID from the post
message structure.

Fixes #539, #564 and #568.

Closes #342

Closes #587

Only personal blogs from non-contacts can be removed.
This also adds integration tests that check if blogs can actually be removed.

Closes #579

When we clear a list in onPause(), the proper behavior is that the empty state
message is shown, because the list is indeed empty.

However, we will reload the list content again in onResume(),
so the fix chosen in this commit is to force showing the progress bar
right after clearing the list.

Closes #576

This refactors the current SharingStatusActivity
so it can be used for forums and blogs.

This commit refactors the code for sharing forums,
so it can be used for sharing blogs as well.

It does not yet include code for responding to blog invitations.

Closes #571

Closes #517

Closes #559

This also adds a setting allowing people to turn blog post notifications off.

Closes #488

Closes #415